1. Field of the Invention
This invention relates to an improved helical scan magnetic recording device. More particularly, it relates to a flying head helical scan recording device which is provided with a lobed rotor for enhancing the dynamic air bearing in the vincinity of the rotating head.
2. History of the Art
The desirability of recording and replaying high speed information signals from television and from high speed digital computers has resulted in increasingly sophisticated magnetic recording devices. Conventional longitudinal scan magnetic recording devices are adequate to record and replay relatively low frequency signals in the audio range. But such devices are not adequate for recording information signals in the video frequency range because it is not practical to drive them to the high linear tape speeds required for use at such frequencies.
As disclosed in U.S. Pat. No. 2,773,120 issued to E. E. Masterson, helical scan magnetic recording devices overcome the necessity for high linear tape speeds. A typical helical scan device comprises a relatively wide magnetic tape, a pair of cylindrical support mandrels, a rotor and a magnetic head (recording or replaying) disposed upon the rotor. The mandrels are axially aligned and positioned end-to-end with a slight gap between them, and the rotor is aligned on the common mandrel axis to rotate in the gap between the two mandrels.
In operation, the wide magnetic tape is wrapped helically about the support mandrels with its edges abutted across the gap between the cylinders, and the rotor carries the magnetic head around the gap to scan a diagonal track on the tape. In such a device, the rotor speed can greatly exceed the tape speed, thereby permitting greatly increased scanning rates.
Although a variety of improvements have been made in the basic helical scan magnetic recording device, excessive wear of the tape and magnetic head remains one of the most serious problems in high-speed devices. Tape wear due to frictional contact between the tape and the cylindrical mandrels has been minimized by the provision of various types of air film bearings between the mandrels and the tape. For example, U.S. Pat. No. 3,139,489, issued to R. T. Lamb, discloses the provision of an air film bearing by the introduction of pressurized air between the mandrel and the tape. Alternatively, U.S Pat. Nos. 3,333,753 and 3,404,241 provide dynamic air film bearings by rotating one or more of the mandrels. The rotating mandrel drags air under the tape, and this air forms a lubricating film.
As may be readily appreciated, a major problem in a high speed helical scan recording device is the interaction between the magnetic tape and the rapidly moving magnetic head. In devices using contact heads, the rotating head tends to drive ripples through the surface of the tape both ahead of it and behind it and to produce excessive wear. U.S. Pat. No. 3,247,329 issued to Yasho Tomita purports to alleviate the ripple problem by the provision of guide wenders on either side of the magnetic head for elevating the tape into straight-line contact with the head. This provision does not, however, solve the basic problem in contact devices, namely that of wear.
The problem of minimizing wear between the tape and the magnetic head has led to the development of flying head recording devices in which the magnetic head need not contact the tape in order to read or record. Rather the magnetic head is designed to "fly" in close, non-contact adjacency (50 microinches or less) to the tape during the predominant portion of its scanning cycle. In such devices the spacing between the tape and the head is critical. Relatively small perturbations in the tape support can cause the tape to travel beyond the sensing range of the head or to crash onto it.
One approach to providing the augmented tape stability required for flying head helical scan recording devices involves providing the rotor with an air bearing film. U.S. Pat. No. 3,840,894 issued to P. J. Arseneault, for example, describes an arrangement wherein the rotor is wide compared to the width of the magnetic head and is provided with an air bearing to provide a stable platform for the magnetic tape along the path of the rotating head.
A second approach to providing augmented tape stability involves the use of a precisely shaped rotor which protrudes beyond the support mandrels to provide a dynamic air bearing and to cause the tape to be locally stressed in the longitudinal region surrounding the magnetic head. U.S. Pat. Application Ser. No. 488,341, filed July 15, 1974, by G. W. Baumann et al., and assigned to applicants' assignee, discloses such an arrangement wherein a constant radius rotor protruding radially beyond the mandrel ends aerodynamically stresses the tape and yet permits the head to fly in microinch adjacency to it.